Xerographic plate comprising a protective coating of a resin mixed with a metallic stearate



March 25, 1969 R.J.JOSEPH ETAL 3,434,832

XEROGRAPHIC PLATE COMPRISING A PROTECTIVE COATING OF A RESIN MIXED WITH A METALLIC STEARATE Filed Oct. 30, 1964 INVENTORS ROBERT J. JOSEPH wn. IAM {MURPHY BY 4 A TTORNE VS United States Patent US. Cl. 96-1.5 22 Claims ABSTRACT OF THE DISCLOSURE A photoconductive member and process of making and using same wherein said member comprises a photoconductive layer overlying an electrically conductive support member and overlying the photoconductive layer a protective overcoating comprising a mixture of film-forming material and metallic stearate.

This invention relates to xerographic plates used in xerography; in particular, to reusable plates having a protective overcoating.

In the process of xerography employing a reusable xerographic plate, the plate is uniformly electrostatically charged in darkness and then selectively discharged by exposure to an optical image of the original subject to be copied. The resulting latent electrostatic image is developed into a visible image by dusting the xerographic plate with finely divided electrostatically attractable toner particles. After the visible toner image is transferred from the plate to a permanent support, such as paper, the plate typicaly passes through a cleaning station where remaining toner particles are removed so that the plate may be recycled in the xerographic process. Plate cleanin is ordinarily accomplished by means of a soft brush or cleaning web which physically removes the toner particles.

The property of photoconductivity is the essence of the xerographic plate. That is, the plate must retain an electrostatic charge in darkness but must become effectively discharged in areas exposed to activating radiation. This property is provided by a thin layer of photoconductive material such as selenium, which is applied to a conductive support member to constitute the xerographic plate.

It is well-known in the art that a reusable xerographic plate need not be in the form of a flat member. Instead, for example, it may be in the form of a cylindrical drum, as is customarily the case in automatic copying machines. For purposes of this specification, the term xerographic plate is intended to encompass the imaging member comprising a photoconductive layer overlying a conductive backing, whether in the form of a flat member or cylindrical drum.

From the standpoint of practicality, a xerographic plate must be recycled many times in the xerographic process. Consequently, the photoconductive layer is subjected to considerable destructive abrasion, especially in the cleaning process. The electrical properties of the photoconductive layer impose certain limitations as to its acceptable thickness. Generally, the photoconductive layer must be extremely thin, and since it is also highly desirable to maintain the uniformity of this layer, an unprotected plate is susceptible to deterioration from abrasion of the photoconductive layer. It is, therefore, highly desirable to protect the layer to prolong its useful life.

Protective overcoatings are known to be useful for increasing the abrasion resistance of a xerographic plate. For example, xerographic plates including a thin protective coating of a resinous polyvinyl acetal are disclosed 3,4'34832| Patented Mar. 25, 1969 in Us. Patent No. 2,860,048 to Deubner. Overcoating layers of various resins, waxes and hydrocarbons are also disclosed in US Patent No. 2,901,348 to Dessauer et al.

The xerographic plate described in the present specification includes a new and superior overcoating to achieve the objects of this invention, one of which is an improved abrasion-resistant xerographic plate. The overcoated xerographic plates described herein have additional desirable properties which are also objects of this invention, includmg: improved adhesion of the overcoating to the photoconductive layer; reduced moisture sensitivity; and, improved cleanability. Thus, in accordance with this invention there is disclosed a superior xerographic plate capable of bein recycled many times in the xerographic process and capable of practicable use under adverse humidity conditions.

Briefly summarized, the present invention comprises a xerographic plate made of a photoconductive layer overlymg a conductive support member, and overlying the photoconductive layer, a protective overcoating comprising a mixture of film-forming material and metallic stearate. In the preferred embodiment, the superior plate comprises a photoconductive layer of vitreous selenium overlying an aluminum support member and a protective overcoating made by mixing nitrocellulose with zinc stearate in a suitable thinner (comprising a combination of solvents), and then applying this mixture to the surface of the selenium layer. The preferred embodiment is not limited with respect to a form of nitrocellulose of particular viscosity or solubility as these varying properties can be accommodated by the amount and type of solvents employed. For example, if the solvents to be used are alcohols or ester-alcohol mixtures, then commercially available nitrocellulose referred to as soluble nitrocellulose may be used. This is understood to have a nitrogen content in the range of l0.512.2%.

In the following detailed description. of our invention, reference is made to the accompanying drawing whih illustrates a three-layer xerographic plate.

In the drawing, xerographic plate 10 includes a photoconducting insulating layer 11 overlying a backing member 12, and a protective overcoating layer 13 covering and protecting at least the photoconductive insulating layer. The backing member is relatively electrically conductive, and typically comprises a metallic plate. Optionally, a glass plate or other non-conductive member of desired structural properties may be used by incorporating a conductive coating, such as tin oxide. Fibrous material, rendered sufiiciently conductive by the addition of a material such as water, particles of carbon, metal or the like, are also usable. Photoconductive insulating layer 11 is of suitable photoconductive material known to the art, preferably in the form of a continuous layer, Typical photoconductors include vitreous selenium, sulphur, anthracene, mixtures of selenium and sulphur, Zinc oxide, and the like. According to one specific embodiment of the invention, the combination of photoconductive layer 11 and backing member 12 may be a xerographic plate having a coating of vitreous selenium on a backing plate of aluminum.

Layer 13 comprises a mixture of a film-forming material and a metallic stearate which must be sufficiently insulating so that there will be no appreciable lateral migration of charge during the xerographic processing steps yet conductive enough so that there will be no appreciable build up of charge across this layer during repetitive xerographic processing steps. Also, layer 13 must permit charge to operate through and effectively reach the photoconductive layer. The film-forming material may comprise any suitable polymer. Typical polymers include, amon others: cellulose, especially nitrocellulose and ethylcellulose; lacquers; ureaformaldehyde resins; medium hard para-sulfonamide resins; alkyd resins; silicone resins; acrylic ester resins.

Any suitable metallic stearate can be used to impart the improved characteristics already mentioned without detracting from the electrical properties of the xerographic plate. Because of their observed superior resistance to abrasion, reduced moisture sensitivity, improved cleanability, and imaging qualities, protective layers including any one of the following materials are especially preferred: zinc stearate, calcium stearate, magnesium stearate, cadmium stearate, lbarium stearate, lithium stearate, lead stearate, iron stearate, sodium stearate, aluminum stearate, nickel stearate, cobalt stearate and copper stearate.

According to a preferred method of preparation, a xerographic plate is first prepared by applying a photoconductive insulating coating to a conductive backing by vacuum evaporation, spraying, or other suitable coating process. This intermediate member is then further processed by applying to it a thin film of the protective material.

The protective material for overcoating the intermediate member is prepared by dissolving the film-forming material and metallic stearate in a suitable solvent bythoroughly mixing agitating, or mechanicall blending the constituents. Typical solvents include: acetone, butyl acetate, butyl alcohol, butyl cellulose, Cellosolve acetate, ethyl ectate, ethyl alcohol, heptane, isopropyl acetate, isopropyl alcohol, methyl ethyl ketone, methyl isobutyl ketone, toluene, xylene, and their equivalents.

Although any suitable coating process may be employed, spray coating is the preferred method for applying the protective material to the xerographic plate. Moreover, superior results are achieved by spraying a solution of protective material made with a thinner comprising a combination of solvents for the film-forming material rather than a single solvent. This is particularly important when the film-forming material comprises a mixture of components having various solubility properties. For example, lacquers may comprise high polymers, resins and plasticizers, and, therefore, require a combination of solvents.

Optimum results with spray coating also requires that the solvents be present in the thinner in the proper proportions. As the protective film dries, the ratio of the solvents changes because of the more rapid evaporation of its lower boiling point components. Again using lacquer as an example: if the final solvent to evaporate is not a solvent for the high polymer, the high polymer may precipitate; if it is not a solvent for the resin, turbidity may result.

Because a relationship has been observed between the ability of overcoated plates to discharge under illumination and the viscosity of the protective material when applied to the xerographic plate, a preferred range of viscosity has been established, viz., 16-19 Zahn #2. Viscosity is conveniently adjusted by controlling the amount of solvent used; adding more or allowing evaporation of excess, as indicated by standard viscosity measurements, prior to the spray coating operation.

Accordingly, optimum results, especially in terms of the electrical properties of the overcoated plate, are achieved in practicing this invention when the abovestated factors are taken into consideration in preparation of the protective material.

The following specific examples of the preparation of the overcoated plates described herein are presented to more fully describe the present invention. Accordingly, they are intended as illustrative only, and should not be interpreted as limiting the scope of the present invention.

EXAMPLE I Using a high-speed mechanical blender, 18.5 grams of nitrocellulose (having a one-half second viscosity) and .38 grams of zinc stearate are dissolved in a thinner comprising (by weight): methyl isobutyl ketone, 34.28%; butyl Cellosolve, 4.28%; ethyl alcohol, 2.15%; isopropyl alcohol, 2.15%; toluene, 57.14%. Sufficient thinner is used to yield a solution having a viscosity in the range 1619 seconds (Zahn #2 cup.). A thin protective layer between .00005 and .00025 in. thick is formed on a standard xerographic plate (consisting of a thin coating of vitreous selenium on one side of an aluminum support) by spraying the solution onto the plate, at approximately 75-80 F., by means of a DeVilbis Automatic Spray Gun, Model P-AGA-571; nozzle No. 29; air pressure 40 lbs. sq. in. The spray gun nozzle is positioned 57 inches from plate during the spraying operation. After curing for 24 hours the plate is usable to make visible toner images by means of conventional xerographic equipment and developer material such as that described in US. Patent No. 2,618,551 to Walkup.

The imaging process comprises: uniformly charging the plate in darkness with the positive electrostatic charge; projecting an optical image to be reproduced, thereby forming a latent electrostatic image; cascading over the plate a developer mix of finely-divided electrically attractable toner and grossly larger carrier particles; electrostatically transferring the toner image thereby formed to a sheet of paper; and, heating the transferred image to form a permanent visible record. Excellent Xerographic copies can be made on the plateovercoated in the manner described, and the plate is capable of being recycled many thousands of times in the xerographic process.

EXAMPLE II A tank is filled with a solution of 1 part of zinc stearate 24 parts of nitrocellulose in thinner (as in Example I) adjusted to a viscosity of 18 seconds (Zahn #2 cup.) and heated to a temperature of 75 80 F. The plate to be overcoated, comprising a coating of vitreous selenium on an aluminum backing, is lowered into the tank to a depth so that all but approximately inch of the plate is immersed. After thermal equilibrium the plate is slowly withdrawn to produce a coating of protective material having a thickness of 23 microns. The thickness of the overcoating is governed by the rate of withdrawal and is determined by means of interference fringes at the beginning of the overcoating. The overcoated plate is allowed to dry and cure for 36 hours before it is used in the xerographic process. Excellent copies are made by seqentially electrostatically charging the plate in'darkness, selectively discharging the plate by exposure to an optical image, dusting the plate with finely-divided pigmented powder, electrostatically transferring the powder image thereby formed to a paper sheet, and heat fusing the transferred image to produce a permanent visible record.

EXAMPLE III In the manner described, a standard xerographic plate is coated with protective material comprising a solution of: one part (by weight) of zinic stearate, twenty-six parts of urea-formaldehyde resin (marketed by Rohm and Haas Co., under the mark Uformite FZOOE) and thirty-seven parts of nitrocellulose /2 sec. viscosity) dissolved in a thinner (as in Example I) to yield a solution having a viscosity of approximately 18 seconds (Zahn #2 cup.). The plate exhibits excellent abrasion resistance and produces Xerographic copies of excellent quality.

EXAMPLE IV A standard xerographic plate is coated with protective material prepared in the manner described and comprising a solution of: 16 parts (by weight) of zinc stearate and 9 parts of nitrocellulose in thinner (as in Example I) to yield a viscosity in the range 16-19 seconds (Zahn #2 cup.). Although readable, xerographic copies produced with this plate are characterized by noticeably reduced density. The plate, however, is exceedingly abrasion resistant.

EXAMPLE V A protective coating having a thickness of 2-3 microns is applied to a standard xerographic plate in the manner described, the coating material comprising a solution of: one part (by weight) of zinc stearate and 19 parts of soluble nitrocellulose in thinner (as in Example I) to yield a viscosity in the range 16-19 seconds (Zahn #2 cup.). Xerographic copies produced with this plate are of excellent quality, and the plate exhibits excellent abrasion resistance characteristics.

EXAMPLES VI-X A series of xerographic plates are overcoated in the manner described in Example V, except that lead stearate is substituted for zinc stearate in Example VI; lithium stearate is substituted for zinc stearate in Example VII; cadmium stearate is substituted for zinc stearate in Example VIII; calcium stearate is substituted for zinc stearate in Example IX; and, sodium stearate is substituted for zinc stearate in Example X. Each of the plates exhibits excellent abrasion resistance; xerographic copies of excellent quality can be made therefrom.

EXAMPLES Xl-XVI Plates of improved abrasion resistance and capable of excellent xerographic image quality are made as in the series of Examples V-X except that Examples XI-XVI comprise a series of plates each overco-ated with a protective coating including 49 parts of nitrocellulose and one part of zinc stearate, lead stearate, lithium stearate, cadmium stearate, calcium stearate, and sodium stearate, respectively.

EXAMPLE XVII In the manner described, a standard xerographic plate is coated with protective material comprising a solution of thinner (as in Example I), 3.25 grams of magnesium stearate and 18.5 grams of nitrocellulose /2 sec. viscosity). The plate overcoated in this manner exhibits even greater wearability than that of Example I, but xerographic copies made therefrom are of slightly diminished quality owing to somewhat reduced density.

In practicing the present invention, it has been found that highly abrasion-resistant plates capable of producing good xerographic toner images can be made with protective overcoatings comprising from 0.5% to about 64% metallic stearate, based on weight of protective material exclusive of solvents. Although plate wearability increases with increased metallic stearate content, image density is gradually adversely affected if the proportion of stearate is increased beyond a certain level. Thus, for practical purposes, it is preferred that the metallic stearate comprise between 1% and of the protective layer.

Plate wearability is also a function of the thickness of the protective layer. Plates of the kind disclosed herein having protective layers up to about 6.5 microns thickness are satisfactory for use in xerography, but the preferred range, in view of electrical properties most suitable for ordinary use, is between 1 and 3 microns.

In view of these practical considerations, and the excellent results achieved, the preferred embodiment of this invention comprises a selenium xerographic plate overcoated with a protective layer 1-3 microns in thickness and comprising about 5% zinc stearate with about 95% nitrocellulose.

Although specific embodiments have been disclosed, it is not intended that these limit the scope of the present invention. The claims appended hereto should, therefore, be interpreted broadly.

What is claimed is:

1. A photoconductive member comprising:

(a) a photoconductive layer overlying an electrically conductive backing member; and

(b) overlying the entire image making portion of said photoconductive layer a protective film comprising a film-forming resin and at least one metallic stearate.

2. A photoconductive member according to claim 1 wherein said protective film contains Zinc stearate.

3. A photoconductive member according to claim 2 wherein the total amount of metallic stearate in the protective film constitutes from about 0.5% to about 64%, by weight, of the protective film.

4. A photoconductive member according to claim 3 wherein the total amount of metallic stearate in the protective film constitutes from about 1% to about 10%, by weight, of the protective film.

5. A photoconductive member according to claim 4 wherein the protective film is from about 0.5 microns to about 6.5 microns thick.

6. A photoconductive member according to claim 1 wherein said film-forming resin comprises a urea-formaldehyde resin.

7. A photoconductive member according to claim 1 wherein said film-forming resin comprises nitrocellulose and wherein said metallic stearate is selected from the group consisting of zinc stearate, calcium stearate, magnesium stearate, cadmium stearate, barium stearate, lithium stearate, lead stearate, iron stearate, sodium stearate, aluminum stearate, nickel stearate, cobalt stearate, copper stearate and mixtures thereof.

8. A photoconductive member according to claim 7 wherein said protective film contains zinc stearate.

9. A photoconductive member according to claim 7 wherein said protective film contains magnesium stearate.

10. A photoconductive member according to claim 7 wherein said protective film contains calcium stearate.

11. A photoconductive member according to claim 7 wherein said protective film contains cadmium stearate.

12. A photoconductive member according to claim 7 wherein the total amount of metallic stearate in the protective film constitutes from about 0.5% to about 64%, by weight, of the protective film.

13. A photoconductive member according to claim 12 wherein the total amount of metallic stearate in the protective film constitutes from about 0.5% to about 10%, by weight, of the protective film.

14. A photoconductive member according to claim 13 wherein said protective film comprises from about 0.5% to about 10% by weight, zinc stearate.

15. A photoconductive member according to claim 14 wherein said photoconductive layer comprises amorphous selenium.

16. A photoconductive member according to claim 15 wherein the protective film is from about 1 micron to about 3 microns thick and comprises, by weight, from about 1% to about 10% zinc stearate and from about to about 99% nitrocellulose.

17. A xerographic process comprising the steps of:

(a) providing a photoconductive member according to claim 1;

(b) uniformly electrostatically charging said member;

(c) exposing said charged member to a pattern of activating electromagnetic radiation to form a latent electrostatic image; and

((1) developing said latent electrostatic image with electrically attractable marking material.

18. A xerographic process according to claim 17 including after said developing step the step of transferring at least a portion of said marking material image to a transfer surface and then repeating the charging, exposing and developing steps at least one more time.

19. A xerographic process comprising the steps of:

(a) providing a photoconductive member according to claim 3; (b) uniformly electrostatically charging said member; (c) exposing said charged member to a pattern of 7 8 activating electromagnetic radiation to form a latent 22. A xerographic process according to claim 21 inelectrostatic image; and cluding after said developing step the step of transferring (d) developing said latent electrostatic image with at least a portion of said marking material image to a electrically attractable marking material. transfer surface and then repeating the charging, expos- 20. A xerographic process according to claim 19 including after said developing step the step of transferring at least a portion of said marking material image to a 5 ing and developing steps at least one more time.

References Cited transfer surface and then repeating the charging, exposing UNITED TATES PATENTS and developing steps at least one more time. 2,803,542 8/ 1957 Ullrich 96-1 21. A xerographic process comprising the steps of: 10 3 33 45 19 4 Kimble et 1 9 .4 (a) providing a photoconductive member according to 3,207,601 9/1965 Giaimo 961 claim 5; 3,251,686 5/1966 Gundlach 961 (b) uniformly electrostatically charging said member;

(6) exposing said charged member to a pattern of NORMAN y Examineractivating electromagnetic radiation to form a latent 15 JOHN COOPER HI Assistant Examine.- electrostatic image; and

(d) developing said latent electrostatic image with US. Cl. X.R.

electrically attractable marking material. 96-1 

